单项选择题

What is the most important factor for a rocket to escape from the Earth ______

A．To travel as fast as it can.
B．To overcome the Earth gravity.
C．To get away from the air resistance.
D．To have strong power in its operation.
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单项选择题
Why is it not necessary to provide powered flight in space all the time ______
A．Few spacecrafts complete their flight operations.
B．The Earth's gravity cannot be overcome.
C．No further power is needed after the escape speed is reached.
D．"Free fall" takes the place of the Earth's gravity pull.
单项选择题
When a bicycle coasts downhill, it ______.
A．runs along the hillside
B．does not need any further power
C．runs freely to all directions
D．does not meet with air resistance
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